trommel

A trommel is a cylindrical rotating screen used to sort and size ore, coal, or aggregates by passing material over or through its perforated or mesh surface. The drum, typically 1 to 3 meters in diameter and 2 to 5 meters long, is mounted at a slight incline and rotates on bearings at 15 to 20 revolutions per minute. As the drum turns, finer material falls through the perforations while oversize material travels along the interior and exits the discharge end, creating a simple but effective gravity-based separation.

The term comes from the German word trommel, meaning drum, and dates back to at least the 18th century in hard rock mining. In its original form, the trommel was a wet screening device; water would be sprayed onto the rotating mesh to help fine material pass through more readily. This wet version, sometimes called a buddle in British mining, could handle clay-heavy ores that would blind a dry screen within hours. Dry trommels became more common in the 19th and 20th centuries, particularly for coal sizing and precious metal recovery.

Practical trommel design involves careful mesh selection. A mine processing lead-zinc ore might use 6 millimeter openings on the upper half of the drum to reject coarse waste, then 3 millimeter openings lower down to separate middling fines. The mesh is typically mild steel wire cloth in older installations, though stainless and manganese steel became standard to resist corrosion and abrasion. Screen blinding, where clay or sticky fines clog the openings, remains the chief maintenance problem; operators would periodically reverse the drum or install rubber balls inside to knock loose the packed material.

Decline and modern context

The term is now largely obsolete in active mining, replaced by more efficient cascade screens, banana screens, and inclined vibrating decks that offer higher throughput and better particle separation. Trommels fell from favor because their low speed (compared to vibrating screens at 1000 rpm or more) meant they could not handle modern mine output rates. However, the mechanism persists in niche applications: small-scale operations, recycling plants sorting mixed waste, and construction aggregate producers still operate trommels built decades ago, often referred to simply as rotating drums rather than by the older technical name.

Understanding trommel design matters when reading historical mining texts, evaluating second-hand equipment, or working on vintage installations in artisanal or small-scale operations. The trommel represents a threshold in mining technology, simple enough to maintain without electricity or complex mechanics, yet effective enough to remain viable at small scales where capital investment and power consumption matter.